feat: manual start exam workflow via teacher dashboard

fix: change Start Now to 10 min prep and add 5 min late entry grace

feat: LeetCode split-pane layout + auto-block after 3 violations

feat: unified nginx proxy deployment

backend

backend

backend

style: Adjust theme contrast

style: Modernize UI with dark glassmorphism design

fix: Correct frontend violation reporting URL

feat: Complete ExamIDE with test cases, auto-judging, scoring, PDF reports, room expiry & Docker